Recently, the study of Hall device techniques has been developed, resulting in the discovery of the current mode of Hall device. For example, as shown in FIG. 1, European patent No. EP0357013 discloses a circuit which employs a method of injecting a current into a Hall axis of a Hall device (HD). In this circuit, the injected current is equal to the output Hall current Ih.sub.o and is then converted to an output voltage indicating the magnetic field strength so that the magnetic field can be measured. However, it is due to the presence of the Hall current that the currents at the two bias contacts of the Hall device may not be equal to each other and may vary with changes in the external magnetic field. This leads to nonlinearity between the Hall current and the magnetic field to be measured and consequently results in inaccurate measurements of the magnetic field. Furthermore, there also is another problem which affects measurement in the circuit. The zero-point of the output voltage thereof is at a value of a portion of the voltage of the power supply divided by the internal resistance of the Hall device and the bias resistors. Therefore, any discreptancy in the internal resistance of the Hall device and any shift of temperature will result in the offset of the output zero-point of the circuit. The symmetrical bias solution presented by Chinese Patent Application. No. 90106846.2 prevents the sum of the currents in the two bias contacts of the Hall device from changing with that of magnetic field so as to provide a linearity between the Hall current and the magnetic field to be measured. The output zero-point of the circuit takes the middle potential of the power-supply voltage of the circuit, and is independent of the internal resistance of the Hall device. Thus the circuit is kept relatively steady. However, in this circuit, since the output zero-point is a floating point, when positive and negative dual-power supplies are employed, any change of the voltage of the power supplies will cause a shift in the output zero-point accordingly.